1. Field of the Invention
The present invention relates generally to energy transfer elements and, more specifically, the present invention relates to energy transfer elements having multiple windings.
2. Backgound Information
FIG. 1 shows an outline schematic diagram of a flyback power supply 101. The basic operation of the flyback converter 101 is well documented and known to one skilled in the art. The primary switch 103 is controlled through a feedback control signal 105, typically but not necessarily from the secondary of the power supply as shown. The energy transfer element or transformer 107 windings have a dot polarity that is used to indicate the phase relationship of the winding voltages. During voltage transitions across the windings, the dot end of the windings are in phase.
FIG. 2 is a schematic of a power supply 201, which expands on the outline schematic of FIG. 1 by representing the parasitic capacitances 209 that exist between the transformer core and electrical earth, the parasitic capacitances 211 that exist between the input and output windings and the transformer core and also the parasitic capacitances 213 that exist between the input and output windings of the transformer. Usually the transformer core is the ferrite core used in the transformer construction to provide a low reluctance path for the magnetic flux coupling input and output windings of the transformer. As noted in FIG. 2, the parasitic capacitance 215 between the output of the transformer and electrical earth in some cases maybe be short circuited depending on the application and or the way in which the electrical noise measurements are made.
During the normal operation of the power supply 201, the voltages across both input and output windings of the transformer 207 transition in accordance with the standard flyback power supply operation. These transitions generate displacement currents in the electrical earth through the various parasitic capacitances 209, 211, 213 and 215 shown. These displacement currents are detected as common mode noise (or emissions) and measured by a piece of test equipment called a Line Input Stabilization Network (LISN). The configuration and connection of this equipment is well documented and known to one skilled in the art.
FIG. 2 also highlights capacitor Cy 217 which is a Y-capacitor, that is commonly used in switching power supplies to reduce the common mode emissions. This component, capacitor Cy 217, provides a low impedance path for displacement currents flowing between input and output windings of the transformer 207, to return to their source without flowing through electrical earth. The currents in capacitor Cy 217 are not detected by the LISN and its use therefore acts to reduce common mode emissions.